The invention relates to an improved electrolytic process for electropolishing of precious metals, base metals, and alloys thereof, and particularly to a process for producing a high surface luster on metallic items by anodic immersion in a non-cyanide solution.
While acidified thiourea solutions have been used for years to chemically dissolve gold, silver, copper and other metals, the presence of an oxidizing agent quickly caused the solutions to break down, producing elemental sulfur and rendering the solution useless. When electrolysis is applied to these solutions in the electrolytic cell, as used for the deposition of metals as in electroplating, a reduction reaction takes place at the cathode and an oxidation reaction at the anode. If both the anodic and cathodic reactions are used to deposit metal at the cathode and dissolve metal at the anode, they are said to be 100% efficient, and no other oxidation or reduction reactions would take place. In electropolishing systems, however, the current applied to the workpiece (the anode) far exceeds what can be used to dissolve metals, and other oxidations reactions are initiated. This is evidenced by the production of oxygen gas which is evolved from the workpiece. The oxygen which is produced at the anode quickly causes the solution to break down in much the same manner as described for chemical dissolution. Another known type of electrolyte, cyanide electrolytes, which has been used to electrochemically dissolve gold, silver, and copper, not only is extremely toxic but is also subject to anodic oxidation. Further, the oxidation process for cyanide electrolytes is far slower than for the unprotected thiourea electrolyte.
It is therefore one object of the invention to provide an improved electropolishing process which employs a nontoxic electroltye and operates quickly and efficiently as a stripper of ferrous metal alloys.
Another object of this invention is to provide a environmentally safe process for the dissolving of metallic silver, gold, copper, and alloys of the same.
Yet a further object of the invention is to provide an electropolishing solution for producing a high lustrous finish of objects made of silver, gold, copper, or alloys of the same, and which is non-toxic and environmentally safe to operate and dispose of when exhausted.
Another object of the invention is to provide an environmentally safe process for removing coating of silver, gold, copper, or alloys of the same from parts made of stainless steel, or other ferrous alloys.